How can I spawn items into a chest, using Command Blocks?

Do

/replaceitem block <x> <y> <z> <slot (if you want to fill the 9th slot of an ender chest with something, change this to slot.enderchest.9)> <item> [data] [dataValue]

This should work: /setblock ~ ~ ~ chest 5 or if it doesnt work try chest 2 {Items:[{id:<item>,Slot:<slot>,Count:<how many>,Damage:0s}]}

Why not just clone a hopper with a certain item that will point to the chest. The only problem I think is you can't really time it well (to me with this idea I filled the chest with one item per tick)

This might be a little roundabout way of looking at it but you could have your chest that already has items in it and attach a hopper that will feed item into it that chest. Then generate a command that will spawn a chest and feed those items of the spawned chest into your untouched chest with the hopper.

here is a link that will auto generate you a command that will generate a chest with custom items in it. With a Example:

/setblock ~1 ~ ~ chest 0 replace { Items: [ { id: "stone", Slot: 4, Count: 1 } ] }

https://minecraftcommand.science/prefilled-chest-generator